As I was typing up my opinion you gave information that I was unaware of. About the wiring...a 12V trolling motor only requires one 12V battery. You can wire it to as many batteries as you want as long as you do it parallel. Two 12V batteries wired in parallel will only supply 12 Volts. By adding the extra battery all you are doing is extending the reserve run time. What that means is you can run longer on two 12V batteries longer than you can run on one 12V battery before you lose power.

To obtain 24V you will wire two 12V batteries in series. if you have a 12V trolling motor and wire it too two 12V batteries in series you will burn up your trolling motor because it is not designed to run on 24V.

You said you last trolling motor was a 12/24V. That means the wiring will be different. Wiring a 12/24 and a straight 24 is two totally different things. I have never owned a 12/24V so I am not up to "par" with the exact wiring. I "think" the 12/24V made the jump to 24V in the way it is wired to the plug in. I "think" the trolling motor wire send power in parallel to the plug in and then when you select 24V it somehow makes a 24V series connection inside the plug in. I may or may not be right so I would get another opinion.

No matter which way you go just make sure you have the right gauge wire and the wiring is correct. if something is wired incorrect you can make a several hundred dollar mistake. If your wires are too small it may run for a while but they will overheat and have the potential to melt.. If my memory is correct 6 gauge wire is what you need 12 or 24 V.

Sounds like you are set on a 12V in the 55 pound thrust with a tiller handle. I am not 100% sure but I believe you can find a tiller that goes higher in thrust pounds than 55 but it will be 24V. Finding something 55 lbs or under set up for 12V should not be a problem...A 20 ft boat will be much easier to handle with 70-80 lbs of thrust...its you coin so you get what you want. I will look online and see if I can find something that might appeal to you.